Why climate change could make Mediterranean atmospheric “meteotsunamis” more common | Horizon: the European magazine for research and innovation
A meteotsunami is a form of tsunami generated by atmospheric conditions, and it can strike any coastline adjacent to a seabed with a long, shallow shelf. They are not as massive, nor as well known, as ordinary tsunamis, which are caused by earthquakes at the bottom of the ocean, and they are more localized. But they can damage property and endanger human life.
A meteotsunami in the port of Ciutadella on the Spanish island of Menorca in 2006, for example, sent yachts crashing into each other and then threw them on the harbor floor as it evacuated, causing tens of millions of euros in damage. Meteotsunami are also featured in local legends – a 16th century meteotsunami in Croatia swept across the bridge that connected the two sides of a village, bringing them back to their senses about a feud that had arisen between them.
Researchers already understand the conditions that trigger such tsunamis, according to Professor Jadranka Šepić, assistant professor and meteorologist at the University of Split, Croatia.
On the coast there must be a descent to a shallow platform up to 100 m deep that juts out at least a few tens of kilometers into the sea. Such a feature is found on the east coast of the States United and in the channel between France and England, for example.
This shape has an effect on the speed of large ocean waves – those that travel long distances rather than the wind-driven rock breakers that hit the shore every few seconds. When these 10 km waves reach this type of plateau, they slow down to a speed of 50 to 110 km per hour, depending on the depth.
Importantly, it can be slow enough to match the speed of the atmospheric gravity waves above them. If the two speeds coincide, and if the two sets of waves then synchronize, the energy of the atmospheric wave siphons into that of the wave in the water, “ and that wave in the sea gets bigger and bigger, kind of like a resonance, ‘said Professor Šepić.
“So on the surface it looks good, but at high levels something more energetic is happening.”
Prof. Jadranka Šepić, University of Split, Croatia
But what causes these atmospheric waves is less well understood, says Professor Šepić. She is leading a project known as SHExtreme to find out the processes behind them and whether they will be more common as the climate changes.
“We know how the atmosphere and the ocean interact… but we want to know what exactly helps these processes in the atmosphere to develop, what kind of larger frame in the atmosphere allows these smaller processes”, a- she declared.
If their incidence increases, she points out, there will be a corresponding increase in the incidence of larger meteotsunamis. In addition, due to the rise in sea level, they will have a wider reach than today.
“The first thing is that they will occur from a higher sea level, so they will be more dangerous,” said Prof Šepić. “But the second thing we need to check is what will happen to these atmospheric waves – will they happen more or less often? If they occur less often, it may happen that these two events cancel each other out … but if we have … more of these favorable atmospheric processes then there is a problem: you have a meteotsunami which is more likely to happen and that will start from a higher level. ‘
SHExtreme collects evidence of these previous waves and compares it with the atmospheric processes underway at the time.
To do this, you need tidal data that was collected by mechanical tide gauges from the 19the century, and is now mainly recorded by digital radar and pressure sensors on the seabed.
For more historical work, however, there was a hitch. Sea level data tables prior to about 2010 record height only hourly. This is too crude a measure because a meteotsunami could roll around, do its damage and be gone in just a few minutes.
For this reason, Professor Šepić had to find the original analog maps – made by a needle oscillating up and down on a rotating cylinder – from which the tables were compiled. She is working on these recordings for the entire Croatian coast. Colleagues, for their part, did the same for Finland.
Since 2010, the International Oceanographic Commission has provided sea level height measurements taken every minute around the world. Professor Šepić’s team therefore gives priority to studying this period, looking for patterns in data such as seasonal distributions and the width of the affected coastline.
So far, the team has shown that Mediterranean meteotsunamis tend to be stronger in summer. Despite calm conditions at ground level, rapid winds of dry air from Africa can pass through the atmosphere at an altitude of some 1,500 meters and this is what seems to trigger the atmospheric waves.
“On the surface it looks good, but at high levels something more energetic is happening,” she said. Testing this theory for the Spanish coasts of the Balearic Islands, she said: “ We were able to show that, if you have this situation in the atmosphere, there is a very high chance that a meteotsunami will occur – this situation. almost always generates these atmospheric waves. ”
The next step is to look to the future. There are many simulations of the behavior of the atmosphere under climate change. For the Balearic Islands, the worst-case scenario of a simulation revealed a 30% increase in the number of days favorable to meteotsunamis due to an increase in the number of days during which there are these high-level winds.
“But the problem is, we only used one climate simulation – it was like a prototype model,” Prof Šepić said. “You have to look at ten or 20 climate simulations.”
She plans to do more simulations in the next few years.
Scientists in the United States believe they have made a breakthrough in forecasting meteotsunamis, at least for Lake Michigan. In April 2018, the seaside town of Ludington was inundated by a wave that damaged boat docks and houses, flooded intake pipes – and could have swept away beachgoers if that had happened a bit more late in the year.
“This is a gap in our forecast,” said Dr. Eric Anderson, an oceanographer with the National Oceanic and Atmospheric Administration’s Great Lakes Environmental Research Laboratory.
His team reported in late March that using photos taken by a local resident and other data, they should, with existing forecasting tools, be able to predict such an event from minutes to hours at a time. advanced.
Prof Šepić says predictions are already in place for the Balearic Islands but are “ often wrong ” because they are based either on statistical chance depending on atmospheric conditions or on models that cannot give accurate forecasts of small-scale atmospheric waves.
SHExtreme can change that. At the very least, said Professor Šepić, “we hope to be able to show which parts of the European coast are most in danger now and also in the future”.
How are our seas changing and rising with climate change and the melting of the Earth’s ice caps? In this three-part series, we look at to the past, present and future of extreme sea level rise. Partly three we will watch how the inhabitants of cities with high flood protection are paradoxically more exposed to the risk of extreme flooding. In Part 1, we looked at what the last interglacial can tell us about extreme sea level rise.
The research in this article was funded by the EU’s European Research Council. If you liked this article, consider sharing it on social media.